DevTools: add 3D layer view to Layer panel

.. sans transforms and colors so far;

BUG=
R=pfeldman@chromium.org

Review URL: https://codereview.chromium.org/22602002

git-svn-id: svn://svn.chromium.org/blink/trunk@156021 bbb929c8-8fbe-4397-9dbb-9b2b20218538